Broadly described, the invention relates to the type of rotary engine in which a rotor has a plurality of radial lobes that traverse the inner surface of a cylindrical wall and cooperate with a plurality of retractible vanes in the cylindrical wall to form expanding and contracting chambers. Rotary engines of this general type are disclosed, for example, in the Mears U.S. Pat. No. 1,192,665, the Thomas U.S. Pat. No. 1,952,149 and in the German Pat. issued to Weber, No. 386,356. The engines of the these prior disclosures are unduly complicated, however, and therefore costly to manufacture and expensive to maintain. The present invention is directed to the need for a simple but highly efficient rotary engine of this general type.